# Negative Voltage

Discussion in 'Electronic Basics' started by Kev, Aug 8, 2004.

1. ### KevGuest

Hey Everyone.

I need to be able to generate a negative voltage from a microcontroller. To
be more detailed, when the microcontrollers output is 1 (high) I want to
generate a positive voltage of 9V. But when the output is 0 (low) I want to
generate a negative voltage of -9V. The positive and negative 9V supply
must be with respect to the microcontrollers ground. The microcontroller is
5V and I have a 9V supply (from a 78L09 regulator). I have no negative
supplies though. The circuit would need to run at speeds of about 70KHz.
The simpler the solution the better.

Thanks.

2. ### KevGuest

One way I thought I might be able to do it was to use a voltage inverter to
turn the 9V into -9V and then use some combination of transistors to switch
between 9V and -9V. Does that make any sense? If it does, could someone
recommend a circuit?

4. ### Byron A JeffGuest

You didn't give the current requirements for the negative supply. Most
charge pump inverters have a max current of 100mA or less.

Here's how I'd do it to keep it simple. To generate the negative voltage
use a standard charge pump, like the cat660, here:

http://www.catalyst-semiconductor.com/documents/660.pdf

The part standard is called the 7660. Other parts in this class of chips
includes the OnSemi 7662 and the Linear Tech LTC1044. Each takes a positive
voltage (+9V in your case) and inverts it.

Then you can use an open ended opamp as your switch. Connect the +9V and -9V
supplies to V+ and V- of the opamp. In its most basic form an opamp will drive
the output so that the two input voltages match. Typically feedback is used
to accomplish this where the output is fed back into one of the inputs.
However in an open ended setup, the output will try to drive the voltage
towards the rails. So the basic rule is that the output will drive positive
as long as the + input is greater than the - input, and vice versa. So
here's how you can set up it. Use a voltage divider to get a 2.5V to the
- input of the opamp. This can be as simple as two equal 100k resistors
connected between +5 and GND. The connect the microcontroller output to the
+ input of the omp amp. When the uC output is +5 the + input is above the
- input so the ompamp output will swing to +9V. OTOH when the microcontroller
output is 0V the + input is below the - input and the opamp output will drive
to -9V. Problem solved.

Be sure to get a rail to rail opamp for the job.

So two ICs, two resistors, and two caps. Not two ;-) much.

BAJ

5. ### Rich WebbGuest

What's your tolerance on that 9 V? Something like a MAX233 will give you
a swing of about +/- 8 V with just one chip and no external components.
It also inverts, of course.

6. ### John PopelishGuest

That is what I was thinking of, but I have no idea if you need any
regulation of the +-9 or if crude copies of the 5 volt regulator input
voltage is okay. What current will load these 9 volt outputs?

7. ### JamieGuest

assuming that you don't have a +/- supply i guess you could
construct a mini op AMP osc with POS feed back then Rectify that via
CAP and Diode.
the output can be directly..
you could use an isolation diode to switch the osc on when the
out goes low.
etc..  